Web tension measuring devices usually consist of load cells mounted underneath the bearings of a roller over which the web passes with a certain angle of wrap of the web around the roller. In locations where this angle of wrap is fixed, such an arrangement is ideal, and sufficient measurement accuracy is usually obtained.
Where tension must be measured in a location adjacent to a coiler, however, which is most often the case when coiling or uncoiling tensions have to be measured, the wrap angle of the web over rollers adjacent to the coil varies as the coil builds up. In such cases the usual solution is to use a second roller placed with its axis parallel to the first roller, which is adjacent to the coiler and a little further away from the coiler than the first roller, so that the web will pass over the second roller with a fixed wrap angle. It is then possible to mount load cells underneath the bearings of the second roller, and the signal output from the load cells is then a direct measure of the web tension, and is not affected by the variation in web wrap angle over the first roller. Such an arrangement is shown in FIG. 1.
This solution is technically satisfactory, but suffers a number of disadvantages. Firstly, the use of a second roller means that the wrap angle of the web is smaller on both rollers than would be the case if a single roller was used. Since such rollers are normally driven by the web as it passes over them, skidding of the web over the rollers is much more likely--such skidding can cause marking of the web surface, and can even cause drive instability if line speed measuring devices are driven from one of these rollers.
Secondly, the initial cost of the equipment increases not only by the cost of the second roller, but also by the cost of the additional support structure and foundations required to support it.
Thirdly, in the case of reversing machines where the web traverses back and forth between the coilers, as is true of reversing rolling mills, the loss of material at the ends of the coil is in direct proportion to the distance between the machine and each coiler. Since the insertion of the second roll inevitably increases the distance between machine and coiler, there is a resultant yield loss for every coil processed by such a machine.